bug#35498: 26.2; doc string of `isearch-regexp-function'

[Drew Adams <drew.adams@oracle.com>]

The doc string should describe the possible values and their
meanings/behaviors.  Currently it describes only a function as value.

Searching isearch.el it seems that this variable can be any of the
values possible for `search-default-mode'.  Please describe each of the
values for `isearch-regexp-function'.

Even when the value is a function the description could be better,
saying that it must accept a string as its first arg and must return a
string.  That can be surmised from saying that it converts a plain
search string to a regexp [search string], but it could be clearer.

Also, "plain search string" and "regexp" are unclear here.  The
distinction is presumably (?) that the former contains no regexp special
chars?  Or does the return value need to be a _valid_ (complete) regexp,
and does the function raise an error if that's not possible?



In GNU Emacs 26.2 (build 1, x86_64-w64-mingw32)
 of 2019-04-13
Repository revision: fd1b34bfba8f3f6298df47c8e10b61530426f749
Windowing system distributor `Microsoft Corp.', version 10.0.17134
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''